The SCARA (Selective Compliance Assembly Robot Arm) type manipulator apparatus refers to an assembly robot having two jointed links pivoting about parallel vertical axes. An embodiment of the SCARA type manipulator apparatus is illustrated and described in detail in U.S. Pat. No. 4,392,776, entitled "Robotic Manipulator Structure" which is assigned to the Westinghouse Electric Corporation. The assignee of the present invention is a wholly owned subsidiary of the Westinghouse Electric Corporation. A novel embodiment disclosed in the above-referenced U.S. patent employs a chain drive system to effect movement of the articulated arms in the XY plane. While the chain drive system represents a workable implementation of the novel manipulator apparatus, practical limitations in the chain drive system adversely affect its utilization as a commercial product. The deficiencies of the chain drive system are primarily due to stretching of the chain over a period of time coupled with the lack of adequate tensioning of the chain with respect to the drive sprockets. The stretching of the chain results in mechanical instability which is exhibited when the direction of motion of the robot arms is changed.